Good question:
1. Rapid demographic change - Meaning the youth regard moral authority first and foremost. Unlike the boomers who think along the lines of ‘khaata hai to lagata bhi hai’. Most of the higher ups in PMLN are corrupt. The youth voters know they are all crooks, the only thing left is for them to go to jail.
2. PMLN is a dynastic party. Senior leaders do not stand a chance even to become Chief ministers. Only way is for them to become samdhis, sandoos, salas, bhenois etc.
3. The young members of the party are not given any chance. It could be related to no sane youth would like to be associated with the party. Hence, no quality people with leadership skills.
4. Only focus on Punjab. PMLN has effectively become a provincial party.
5. Tainted past. Ruling for several decades yet no result. 90s was a lost decade. Post Musharraf also same. No marked improvement in economy or human resource.
6. Last and not the least. The leader is corrupt, second tier mostly corrupt, mid level has a good number of corrupt elements. Not even counting the murderers.